Scheduling non-preemptive hard real-time tasks with strict periods

Partial answers have been provided in the real-time literature to the question whether preemptive systems are better than non-preemptive systems. This question has been investigated by many authors according to several points of view and it still remains open. Compared to preemptive real-time scheduling, non-preemptive real-time scheduling and the corresponding schedulability analyses have received considerable less attention in the research community. However, non-preemptive scheduling is widely used in industry, and it may be preferable to preemptive scheduling for numerous reasons. This approach is specially well suited in the case of hard real-time systems on the one hand where missing deadlines leads to catastrophic situations, and on the other hand where resources must not be wasted. In this paper, we firstly present the non-preemptive model of task with strict period, then we propose a schedulability condition for a set of such tasks, and finally we give a scheduling heuristic based on this condition.

2009

In this paper we study hard real-time systems composed of dependent strictly periodic preemptive tasks in the monoprocessor case. Although preemptive scheduling algorithms are able to successfully schedule some systems that cannot be scheduled by any non preemptive scheduling algorithm, the cost of preemption may not be negligible. Therefore, its exact cost has to be explicitly considered in the schedulability conditions in order to avoid wasting resources and provide safety in terms of guaranteeing the right behavior of the system at run-time. Because we are interested in hard real-time systems with precedence and strict periodicity constraints where it is mandatory to satisfy these constraints, we have already shown in a previous work how to tackle this problem for systems composed of harmonic tasks. Two main contributions are presented in this article. First, we generalize our previous results to the case of systems with periods that are not necessarily harmonic. Second, we provide a necessary and sufficient schedulability condition which takes into account the exact number of preemptions for a system with such constraints when no idle time is allowed.

IEEE Symposium on Emerging Technologies and Factory Automation, ETFA, 2006

Classical off-line approaches based on preemption such as RM (Rate Monotonic), DM (Deadline Monotonic), EDF (Earliest Deadline First), LLF (Least Laxity First), etc, give schedulability conditions but most of the time assuming on the one hand that all the tasks are independent, and on the other hand, that the first instances of all tasks are released at the same time. We are interested in hard real-time systems subject to precedence and strict periodicity constraints, i.e. such that for all instances of each task, the release time and start time are equal. For such systems, it is mandatory to satisfy these constraints. In this paper we give non-schedulability conditions in order to restrict the study field of all systems of tasks to only potentially schedulable systems.

Classical approaches based on preemption, such as RM (Rate Monotonic), DM (Deadline Monotonic), EDF (Earliest Deadline First), LLF (Least Laxity First), etc, give schedulability conditions in the case of a single pro- cessor, but assume the cost of the preemption to be negli- gible compared to the duration of each task. Clearly the global cost is difcult to determine accurately because, if the cost of one preemption is known for a given proces- sor, it is not the same for the exact number of preemptions of each task. Because we are interested in hard real-time systems with precedence and strict periodicity constraints where it is mandatory to satisfy these constraints, we give a scheduling algorithm which counts the exact number of preemptions for each task, and thus leads to a new schedu- lability condition. This is currently done in the particular case where the periods of all the tasks constitute an har- monic sequence.

2008 Euromicro Conference on Real-Time Systems, 2008

Task period adaptations are often used to alleviate temporal overload conditions in real-time systems. Existing frameworks assume that only task periods are adjustable and that task deadlines remain unchanged at all times. This paper formally introduces a more general real-time task model where task deadlines, which are less than or equal to task periods, are functions of task periods. This tight coupling between task deadlines and task periods has been discussed in a recent work in control systems and presents a novel real-time scheduling challenge. To solve the period and deadline selection problem, this article identifies a feasible period-deadline combination and proposes a heuristic, which iteratively adjusts task periods and deadlines in such a way that the task set becomes schedulable. Experimental results show that the heuristic finds a solution to the period and deadline selection problem over 73% of the time, using less than three search iterations. When it is unable to find a solution to the problem, the heuristic requires less than 0.02s to run in the worstcase (with at most 100 search iterations).

Proceedings of 2012 IEEE 17th International Conference on Emerging Technologies & Factory Automation (ETFA 2012), 2012

We consider the problem of fixed priority scheduling of non-preemptive strict periodic tasks in conjunction with sporadic preemptive tasks. There are few studies about the scheduling problem combining these two kinds of tasks. Moreover, only few results are available on scheduling non-preemptive strict periodic tasks since their performance analysis gives low success ratios, except in the case of harmonic tasks. Also, strict periodic tasks are of great importance since they are in charge for example of sensors/actuators or feedback control functions which are all critical in feedback control systems. Such tasks must have the highest priorities in order to guarantee a correct behavior of the control system. Preemptive sporadic tasks can be used for non critical functions and have lower priorities.

1999

This paper proposes e cient scheduling algorithms for the joint scheduling of hard aperiodic, sporadic and periodic real time tasks, in systems based on preemptive, xed-priority dispatching. Our scheme guarantees or rejects hard aperiodic real time tasks without any prior knowledge of their attributes, by managing the idle processor capacity dynamically. The method assigns xed priorities to periodic tasks based on the Deadline Monotonic (DM) scheme and analyzes their schedule o -line. We derive closed form solutions for the idle processor capacity process Z(t) within a schedule. In the absence of pending dynamic tasks, periodic tasks execute in their earliest possible schedule S F , called the Fixed-Priority First (FPF). Upon the arrival of a non-periodic task Ja, the scheduler directly determines its admissibility, based on closed form equation of the available processor time in the current schedule, until the deadline of Ja. If Ja cannot be guaranteed under FPF, the scheduler evaluates the idle processor capacity of an alternative schedule S L , where periodic tasks are delayed to execute at their latest possible times, called the Latest Deadline Last (LDL). If LDL o ers su cient idle capacity, the scheduler switches all periodic tasks from FPF to LDL, assigns Ja the lowest priority and admits it into the system. Otherwise, it immediately rejects Ja. We develop the theoretical framework and derive e cient algorithms to compute the idle processors capacity Z(a; b) within a time interval a; b], and maintain it when the schedule is adjusted. The algorithms can also reclaim unused capacity from guaranteed tasks. Our admission control procedure has computational complexity (n) when the non-periodic task queue is serviced in FIFO order, with n periodic tasks. Previously proposed methods have pseudopolynomial time and space complexity. Experimental results show that with n = 160 periodic tasks, the actual computation time for the admission control procedure is less than 90 -secs on a SUN Ultra-Sparc I, 143MHz machine and less than 30 -secs on a SGI Origin 2000, 250MHz workstation. Experiments on well known task sets show overheads which are below 10 -secs even for the slowest machine. The proposed methodology easily extends to algorithms that minimize total task tardiness and number of tardy tasks.

We consider the problem of fixed priority scheduling of non-preemptive strict periodic tasks in conjunction with sporadic preemptive tasks. There are few studies about the scheduling problem combining these two kinds of tasks. Moreover, only few results are available on scheduling non-preemptive strict periodic tasks since their performance analysis gives low success ratios, except in the case of harmonic tasks. Also, strict periodic tasks are of great importance since they are in charge for example of sensors/actuators or feedback control functions which are all critical in feedback control systems. Such tasks must have the highest priorities in order to guarantee a correct behavior of the control system. Preemptive sporadic tasks can be used for non critical functions and have lower priorities.

Real-Time Systems Symposium, …, 1999

Scientia Iranica, 2014

Performability is an important parameter in safety-critical real-time systems. This parameter is defined as the joint consideration of two other important parameters, i.e., reliability and performance. This paper proposes a schedulability condition, which guarantees a desired level of performability in various working conditions, for real-time systems. The basic idea underlining this condition is to select a subset of schedulable tasks and manage their slack times to satisfy a desired performability level. The proposed condition is evaluated on a hard real-time system that employs the Rate-Monotonic (RM) scheduling algorithm and uses the re-execution mechanism to improve the reliability. Evaluation results reveal that by employing the condition, the level of performability of the system is always greater than the desired performability. In addition, it yields on average 1% improvement in the system performability in comparison with traditional schedulability conditions, while the ac...